The present invention relates to method and apparatus for performing measurement/testing of the flying height of read-write head sliders utilized in disk-type data/information recording, storage, and retrieval systems. More particularly, the present invention relates to method and apparatus for performing flying height measurement/testing with increased sensitivity at very low. flying heights on the order of 5xcexc inches or less, e.g., 1xcexc inch or less.
Thin film magnetic and magneto-optical (xe2x80x9cMOxe2x80x9d) recording media are conventionally employed in disk form for use with disk drives for storing large amounts of data in magnetizable form. Typically, one or more disks are rotated on a central axis in combination with data transducer heads. In operation, a typical contact start/stop (xe2x80x9cCSSxe2x80x9d) method commences when the head begins to slide against the surface of the disk as the disk begins to rotate. Upon reaching a predetermined high rotational speed, the head floats in air at a predetermined distance from the surface of the disk due to dynamic pressure effects caused by the air flow generated between the sliding surface of the head and the disk. During reading and recording operations, the transducer head is maintained at a controlled distance from the recording surface, supported on a bearing of air as the disk rotates, such that the head can be freely moved in both the circumferential and radial directions, allowing data to be recorded on and retrieved from the disk at a desired position. Upon terminating operation of the disk drive, the rotational speed of the disk decreases and the head again begins to slide against the surface of the disk and eventually stops in contact with and pressing against the disk. Thus, the transducer head contacts the recording surface whenever the disk is stationary, accelerated from the static position, and during deceleration just prior to completely stopping. Each time the head and disk assembly is driven, the sliding surface of the head repeats the cyclic sequence consisting of stopping, sliding against the surface of the disk, floating in air, sliding against the surface of the disk, and stopping.
It is considered desirable during reading and recording operations, and for obtainment of high areal recording densities, to maintain the transducer head as close to the associated recording surface as is possible, i.e., to minimize the xe2x80x9cflying heightxe2x80x9d of the head slider. Thus, a smooth recording surface is preferred, as well as a smooth opposing surface of the associated transducer head, thereby permitting the head and the disk surface to be positioned in close proximity, with an attendant increase in predictability and consistent behavior of the air bearing supporting the head during motion.
As should be evident from the above, an experimental method for verification and testing of the fly height of the head slider during both the design and production phases of read-write heads for rotating disk magnetic and MO storage media is necessary. At present, three (3) wavelength interferometry, as for example, disclosed in U.S. Pat. No. 5,280,340 to C. Lacey, the entire disclosure of which is incorporated herein by reference, is the most commonly employed technique for direct measurement of fly heights. According to this technique, a test apparatus (e.g., such as manufactured by Phase Metrics, Inc., San Diego, Calif.) is utilized which comprises an optically flat, very smooth, light transparent (e.g., glass), rotating disk, typically coated on a first (e.g., lower or back side) with a very thin (e.g., 0.5-1.0 nm) layer of a perfluoropolyether lubricant, and a means for controllably positioning a head slider at a very small spacing (i.e., flying height or air gap) from the first surface of the disk. As shown in FIG. 1, white light emanating from a suitable source impinges the second (i.e., upper or front side) surface of the disk (illustratively at substantially normal incidence) and is transmitted through the disk. A first portion of the transmitted incident light travels through the air gap d between the first surface of the disk and the head slider, and reflected thereat back through the disk for ultimate receipt by a suitable detector positioned above the second (front side) surface of the disk; whereas a second portion of the transmitted incident light is reflected at the first (back side) surface of the disk back through the disk for ultimate receipt by the detector. The first and second portions of the transmitted light reflected from the head slider surface and from the first (back side) surface of the disk, respectively, are both constructively and destructively combined by interference in the space before the detector to yield a detector output which produces an intensity vs. wavelength pattern, depending upon the spacing (flying height) between the glass disk and the head slider (the preceding assumes that any portion of the incident light reflected from the second or front side surface of the glass disk is small and that any interference effect resulting therefrom is very small due to the thickness of the disk being much greater than the flying height d).
More specifically, and with reference to FIG. 2, the total reflected light intensity vs. wavelength resulting from the constructive and destructive interference of the first and second portions of the reflected incident light is modulated at a specific air gap or flying height d to produce a generally sinusoidally-shaped intensity vs. wavelength pattern having spaced-apart maxima and minima, and is compared with a calibration curve to determine the actual flying height. If incident light of a particular wavelength is utilized for the measurement, a half-cycle of the reflected intensity modulation corresponds to a change in the air gap or flying height d equal to one quarter (xc2xc) of the particular incident wavelength. For example, for yellow/green incident light of 560 nm wavelength, the wavelength spacing between adjacent peaks of reflected light intensity of the intensity vs. wavelength pattern corresponds to a change in air gap or flying height of about 140 nm.
However, unlike typical interferometric measurements, the distances or spacings to be measured in air gap or flying height applications are much less than the wavelengths of the light utilized for the measurement, typically on the order of 25 nm or less; consequently, only a small portion of the peak-to-peak reflected light intensity vs. wavelength modulation pattern can be utilized for flying height measurement. Therefore, in order to maximize the sensitivity of the measurement, it is advantageous for the air gaps or flying heights corresponding to the relatively slowly changing reflected light intensities at the maximum and minimum of the modulation pattern of the reflected light intensity to be far from the air gap or flying height region of interest, where the reflected light intensity is desired to change rapidly with change in air gap or flying height. Further, in order to maximize measurement sensitivity at spacings of 25 nm or less, it is considered essential that overall intensity losses arising from the disk substrate due to, inter alia, internal reflection and absorption within the glass disk and external reflection and scattering therefrom, be minimized.
As indicated above, the continuing requirement for decreased flying heights for obtaining increased areal recording densities of magnetic recording media has necessitated continuing improvements in the sensitivity and accuracy of flying height measurements at very low head-to-disk spacings. However, accurate determination of flying heights below about 25 nm utilizing lubricated optical glass disks, as described supra, have become ever more problematic. While the sources or origins of the difficulties are several and varied, they are, in essence, dominated by the fact that the glass material utilized for the disk is a poor tribological surface for interaction with head sliders which are typically provided at their sliding surface(s) with a wear-resistant coating, e.g., of diamond-like carbon (xe2x80x9cDLCxe2x80x9d). Moreover, in addition to physical damage imparted to the glass disk surface and the head slider due to their intermittent contacting as in the CSS operation described above, the head flying over the lubricant-coated glass disk surface often incurs undesirable lubricant/contaminant buildup, resulting in alteration of the air bearing characteristics and instability of the flying height.
While it is well known that addition of a lubricated hard carbon overcoat (e.g., of DLC) to a rotating disk surface can significantly improve the tribological performance thereof by affording protection against friction and wear induced by contact with the DLC-coated head slider, a difficulty arises in that the optical properties of sputtered carbon (utilized for the DLC overcoat) are such that at the minimum thickness (i.e., about 5 nm) necessary to improve the tribology of the head slider-disk interface, the total change in reflected light intensity at the detector during interferometric air gap or flying height measurement as described above, e.g., for green-yellow light of about 562 nm wavelength, over a fly height range of from near 0 to about 25 nm, is reduced to about one-fourth (xc2xc) of the intensity change obtained with bare (i.e., uncoated) glass. The disadvantageous reduction in intensity change attendant upon the use of glass disks with lubricated DLC overcoats, hence measurement sensitivity reduction, is attributed both to increased internal reflection within the glass disk at the disk/DLC interface due to the poor refractive index (n) match of the two materials, and to an increased amount of light absorption and scattering within the DLC layer.
In addition to the reduction in measurement sensitivity attributable to the above-described optical effects such as absorption and scattering, an additional reduction in measurement sensitivity results from the fact that the composite optical properties of carbon-coated glass substrates are such that the position of the minimum in the intensity vs. fly height curve is shifted into the fly height region of interest. This phase shift is a general property of multi-layer stacks, and must be taken into consideration as a factor affecting measurement sensitivity, hence precision and accuracy.
Accordingly, there exists a need for an improved method, apparatus, and disk means for performing interferometric measurement/testing of the fly height of a read-write head slider over the surface of a light transparent, rotating disk (e.g., of glass) having at least a wear-resistant protective overcoat layer thereon for improving the tribological properties thereof, which method, apparatus, and disk means overcome the drawbacks and disadvantages associated with the prior art means and methodology and affords increased measurement sensitivity at very low flying heights below about 5xcexc inches, while providing full compatibility with all aspects of interferometric air gap or flying height measurement/testing.
The present invention addresses and solves problems attendant upon the use of interferometric techniques for the measurement/testing of very small air gaps or flying heights of read-write head sliders utilized in very high areal recording density rotating disk-based, magnetic data/information recording, storage, and retrieval media and systems, while preserving the essential features of conventional interferometric air gap measurement apparatus and technology. An advantage afforded by the present invention is the ability to fabricate and utilize disks required for fly height measurement which comprise coated glass substrates akin to those utilized for magnetic disks, which coated glass substrates may be prepared by means of techniques and instrumentalities conventionally employed in the manufacture of magnetic recording media. Moreover, the means and methodology of the present invention can be utilized for gap or spacing measurement/testing as may be required for all manner of devices, for example, devices utilizing probe scanning techniques, e.g., Atomic Force Microscopes (xe2x80x9cAFMxe2x80x9d).
An advantage of the present invention is an improved method for performing interferometric measurement/testing of flying heights of read-write head sliders utilized in e.g., magnetic data/information recording, storage, and retrieval.
Another advantage of the present invention is an improved rotatable disk for use in performing interferometric measurement/testing of flying heights of read-write head sliders.
Still another advantage of the present invention is an improved apparatus for performing measurement/testing of flying heights of read-write head sliders of data/information recording, storage, and retrieval systems.
Additional advantages and other aspects and features of the present invention will be set forth in the description which follows and in part will become apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. The advantages of the present invention may be realized and obtained as particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are obtained in part by a method for performing interferometric measurement/testing of the fly height of a read-write head slider over a surface of a rotating disk 10, as shown in FIG. 7, the disk surface having at least a wear-resistant, protective overcoat layer 14 thereon for improving tribological properties thereof, the method comprising transmitting at least a portion of an incident light beam through the rotating disk for reflection by an air bearing surface of the head slider facing the disk surface through an air gap, the method further comprising optimizing the optical properties of the disk surface for enhancing the sensitivity of the interferometric measurement/testing by increasing the intensity of reflected light received by a suitable detector.
According to embodiments of the present invention, the protective overcoat layer 14 comprises a material having a high refractive index from about 1.8 to about 2.4 (at an interferometric measurement wavelength of about 562 nm), e.g., a diamond-like carbon (DLC) material; and the disk surface further comprises a lubricant topcoat layer 22 over the protective overcoat layer 14.
In accordance with a particular embodiment of the present invention, the disk 10 comprises a light transmissive substrate 12 having a high refractive index from about 1.9 to about 2.5, the high refractive index substrate 12 being index-matched to the high refractive index protective overcoat layer 14, whereby loss of reflected light intensity due to internal reflection within the disk 10 is substantially eliminated, or at least reduced, and the phase shift characteristic of multi-layer stacks is minimized, or at least reduced; e.g., the protective overcoat layer 14 comprises a high refractive index, diamond-like carbon (DLC) material and the disk 10 comprises a high refractive index glass substrate 12.
According to another embodiment of the present invention, the disk 20 comprises a light transmissive substrate 18 having a low refractive index from about 1.4 to about 1.6, as shown in FIG. 8, with an underlayer 22 of a material having a very high index of refraction n from about 1.9 to about 2.6 and a very low extinction coefficient k interposed between the disk surface and said high refractive index protective overcoat layer 14, with the primary considerations being both the gain or loss in measurement sensitivity due to the phase shift and intensity loss(es). For example, the very high refractive index, very low extinction coefficient underlayer 22 comprises a material selected from the group consisting of ZnS, SiN, TiO2, ZrO2, Ta2O5, HfO2, TiN, BN, and multi-layer metal structures, with ZnS and SiN presently preferred; the high refractive index protective overcoat layer 14 comprises a diamond-like carbon (DLC) material; and the thickness of each of the protective overcoat layer 14 and underlayer 22 is selected to provide enhancement in reflected light intensity received by the detector for air gaps, hence fly heights of the head slider, of about 5xcexc inches and below, e.g., for air gaps not greater than about 1xcexc inch.
Another aspect of the present invention is a disk 10 for use in an apparatus for performing interferometric measurement/testing of flying heights of read-write head sliders, the disk 10 having a central opening for use with a spindle for rotation about a central axis, the disk 10 comprising:
a substrate 12 comprised of a light transmissive material and including a pair of opposed, smooth, major surfaces; and
a wear-resistant, protective overcoat layer 14 on one of the major surfaces for improving the tribological properties thereof;
wherein the optical properties of the one surface of the disk 10 are optimized for enhancing the sensitivity of the interferometric measurement/testing by increasing the intensity of reflected light received by a detector of said apparatus.
According to embodiments of the present invention, the protective overcoat layer 14 comprises a diamond-like carbon (DLC) material having a high refractive index from about 1.8 to about 2.4 (at an interferometric measurement wavelength of about 562 nm); and a lubricant topcoat layer 16 is provided over the protective overcoat layer 14.
In accordance with an embodiment of the present invention, the light transmissive substrate 12 comprises a glass material having a high refractive index from about 1.9 to about 2.5, the high refractive index glass material being index-matched to the high refractive index DLC material of the protective overcoat layer 14, whereby loss of reflected light intensity due to internal reflection within the disk 10 is substantially eliminated, or at least reduced.
According to another embodiment of the present invention, the light transmissive substrate 18 comprises a glass material having a low refractive index from about 1.4 to about 1.6; and the disk 20 further comprises an underlayer 22 of a material having a very high index of refraction n from about 1.9 to about 2.6 and a very low extinction coefficient k from 0 to about 0.5 interposed between the one surface of the low refractive index glass substrate 18 and the high refractive index, protective overcoat layer 14 of DLC material; wherein the very high refractive index, very low extinction coefficient underlayer 22 comprises a material selected from the group consisting of ZnS, SiN, TiO2, ZrO2, Ta2O5, HfO2, TiN, BN, and multi-layer metal structures, with ZnS and SiN presently preferred; and the thickness of each of the protective overcoat layer 14 and the underlayer 22 is selected to provide enhancement in reflected light intensity received by the detector for flying heights of the head slider between about 0 and about 5xcexc in.
Yet another aspect of the present invention is an apparatus for performing interferometric measurement/testing of flying heights of read-write head sliders, comprising:
a rotatable disk 10 comprised of a light transmissive substrate material; and
means for optimizing the optical properties of one side of the disk 10 for enhancing the sensitivity of the measurement/testing for flying heights between about 0 and about 5xcexc in.
According to embodiments of the present invention, the one side of the disk 10 includes protective overcoat 14 and lubricant topcoat 16 layers thereon.